1. Field of the Invention
The present invention relates to a cooperative localization process and related apparata. In particular, the present invention relates to a process and related apparata that enable estimating the position of radio apparata that do not have access to localization systems, or where these are not working or are ineffective, and in which the acquisition of data for calculating the unknown positions takes place through the cooperation of apparata that are present in the surrounding area.
2. Present State of the Art
In recent years, determining the position of apparata has grown in importance, due both to interest in services known as LBS (Localization Based Services) and to accompany emergency calls with caller position data. Various process and systems have been studied and implemented for determining the position of radio apparata, some of which are based on satellite systems, such as GPS (Global Positioning System), GLONASS (GLObal NAvigation Satellite System), GALILEO (currently being implemented by the EU) and COMPASS (currently being implemented by China), while other are based on terrestrial radio systems, and cellular systems in particular.
The most well-known terrestrial process for determining the position of a terminal are:                identification of the radio cell from which the terminal signal originates;        determination of the area of coverage common to the radio stations that simultaneously receive the terminal's signal;        identification of the radio cell from which the terminal's signal originates combined with an estimate of the distance of the terminal from the radio base station;        triangulation, or rather determination of the directions of origin of the terminal's signal in at least two other radio stations, with calculation of the intersection of the corresponding lines;        multilateration, or rather estimation of the distance of the terminal from at least three radio base stations, the geographic coordinates of which are known, with calculation of the point that satisfies said distances;        determination of at least two different cases with differences in distance between the terminal and the two base stations of a pair of stations, with calculation of the point that satisfies said distance differences.        
Some of the above-listed terrestrial process use the estimate of distances of the terminal from the radio stations, which can be obtained using various techniques:                the ratio between of the received signal and the power of the transmitted signal, whether in the base-mobile direction (in the case of WCDMA, this ratio corresponds to RSCP parameter—Received Signal Code Power), or in the opposite direction;        measurement of the propagation time between transmitter and receiver;        measurement of the round-trip time of the electromagnetic waves from the base station to the terminal (in some systems, including GSM, this can be deduced from the TA (Timing Advance), i.e. the time by which the terminal must anticipate transmission of its signals to make them arrive at the base stations in the established time windows),        any other technique that enables obtaining the same result.        
In other cases, the terrestrial process use three or more distance differences between the terminal and two radio stations and the intersections of the hyperbolae that these differences define. As a rule, these differences are calculated by observing the difference in time that the radio signals employ for travelling along the two different propagation paths (Observed Time Difference, OTD), with variants of the techniques dependent on the process used for synchronising the stations or to take into account the time lag between their clocks.
However, the known solutions indicated above have certain technical problems and drawbacks. In particular, satellite localization systems require a number of satellites to be electromagnetically visible from the apparatus to be localized and entail long, initial acquisition times. With the A-GPS (Assisted-GPS) system, the cell stations provide the terminals with preliminary data on the satellite system with which the terminals can increase GPS signal reception sensitivity and shorten the initial acquisition, but times remain very long when GPS is unavailable because the propagation of the signals transmitted by the satellites is blocked by natural or artificial obstacles.
Similarly, excluding techniques that refer to just one radio station, in many places localization systems based on cellular networks cannot be used because there is not always electromagnetic visibility between the apparatus to be localized and at least two radio stations (required by the above-mentioned, rather imprecise triangulation technique) or three radio stations, needed for hyperbolic or multilateration systems. There again, techniques that refer to just one radio station normally offer inadequate accuracy, and so it must be concluded that in many places both satellite localization systems and terrestrial ones are not available or do not provide an estimate of terminal position with sufficient accuracy.
To compensate these shortcomings, process have been proposed that, for estimating the position of an apparatus that cannot access localization systems, use the position data of other apparata that are in the vicinity. The process described in patent EP1206152 belongs to this category. According to this process, the terminal to be localized has a radio transceiver system that enables it to establish two-way connections with other terminals that are in the vicinity. By means of said radio transceiver system, the terminal to be localized has the nearby terminals send it data regarding their respective positions and estimates the distance of each one of them with an evaluation of propagation attenuation. The terminal to be localized estimates its position on the basis of this data.
In a first embodiment of the process, the unknown position is calculated as the mean of the positions of the terminals that have provided their position. In this case, there is no guarantee on the accuracy of the calculated position, as there is no information on the relative position of the terminal to be localized with respect to those that are in the vicinity, particularly if it is assumed that the terminal to be localized, like the surrounding ones from which it waits for data, has a localization system that it momentarily cannot access. In this case, terminals very close to the one to be localized are probably also unable to determine their own position by means of an external localization system. It is therefore likely that the positions from which the mean is calculated correspond to points further away from the terminal to be localized, and not arranged uniformly around it, but more likely arranged in an irregular and unbalanced manner. Furthermore, the estimate of the distances made on the basis of propagation attenuation can be misleading, especially if conditions are considered where signals are propagated inside buildings or in areas with natural or artificial obstacles. For example, a terminal close by, but separated by a wall or a ceiling can appear further away than a terminal whose signals arrive through a window from a distant external point. By applying the process of patent EP1206152, it is therefore probable that a terminal inside a building calculates that it is on the outside instead.
In a second embodiment of the process of patent EP1206152, the unknown position is calculated with multilateration of the distances and positions of the terminals that have communicated their position data. In this case, it is necessary to have at least three terminals, which know their own position and are electromagnetically visible to the terminal to be localized; the probability of this occurring might not be very high. Then, should three terminals be found in these conditions, as mentioned above, it is likely that they are not close to the terminal to be localized and that the determination of the distance from them by measurement of the power of the signals received from them is misleading. Therefore, the process of patent EP1206152 does not appear to be reliable.